MixedDependencyLockingIntegrationTest.groovy

Enable improved POM support by default This commit makes the experimental flag `IMPROVED_POM_SUPPORT` the default. The flag is still there for backwards compatibility but has effectively no impact. As a consequence, the behavior of improved POM support is now the default, which implies that:

- Maven dependencies packaged as `pom` or `jar` now have derived variants (`compile` and `runtime`) and we properly choose between the variants based on the consumer attributes - platform dependencies using the `platform` and `enforcedPlatform` keywords are enabled

Enabling improved POM support by default is a **breaking change**: there's a risk that resolved dependencies is different, in particular because we will now only include the `compile` dependencies of a POM file whenever the consumer asks for the API variant. There are also some changes in the dependency insight reports due to the use of attribute based matching instead of configuration selection.

Last but not least, this commit is likely to introduce a small performance regression due to attribute based selection.